The present invention relates to automotive-type engine valve control systems and, more specifically, to a system for achieving a rapid warm-up of the catalytic converter during cold-starts to reduce tailpipe emissions.
Conventional automotive engines maintain a fixed lift, duration, and phasing of intake and exhaust valve events. As a result, there is a compromise between the best fuel economy, emission control, and engine power conditions. It is known that objectives such as improved fuel economy, emission control, and other engine output benefits can be realized if the timing of these valve events are varied depending upon the engine operating mode. The present invention is directed to a cold-start system of phase shifting the intake and exhaust valve camshafts with a unique strategy to reduce catalyst warm-up time thereby minimizing tailpipe emissions.
Prior art U.S. Pat. No. 5,588,411 discloses a system for retarding the opening of at least one intake valve during cold-starts to increase the velocity and mixing of the charge. The U.S. Pat. No. 5,228,422 concerns a method for intake valve control in a fuel injected engine to improve air/fuel mixture preparation in relation to cold-starts and warm-up. The U.S. Pat. No. 4,892,067 teaches retarding the intake valve opening to increase turbulence and mixing between fuel and air but does not employ such strategy at engine start-up.
Referring to prior art FIG. 1, a simplified cross-sectional view of an automotive engine cylinder 10xe2x80x2, together with two of its valves, illustrates a known valve-overlap timing sequence. The cylinder inlet valve seat 12xe2x80x2 communicates with an Air/fuel inlet port 14xe2x80x2 while its exhaust valve seat 16xe2x80x2 communicates with its exhaust valve port 18xe2x80x2. Cylinder inlet valve 20xe2x80x2 and exhaust valve 22xe2x80x2 control their respective inlet 12xe2x80x2 and exhaust 16xe2x80x2 valve seats. Reciprocating piston 24xe2x80x2 is shown at its top dead center (TDC) position on completion of its exhaust stroke. In the known valve-overlap timing arrangement, the total valve-overlap crank-angle movement when both inlet and exhaust valves are open simultaneously in the piston TDC region between piston exhaust and intake strokes, is called the valve-overlap period.
The prior art FIG. 5 valve timing diagram shows exhaust valve opening and closing event arc 30xe2x80x2 and intake valve opening and closing event arc 32xe2x80x2. Exhaust event arc 30xe2x80x2 indicates exhaust valve 22xe2x80x2 opening at about 58 degrees before bottom dead center (BDC) and closing at about 10 degrees after TDC. Intake valve arc 32xe2x80x2 indicates intake valve 20xe2x80x2 opening at about 10 degrees before TDC and closing at about 58 degrees after BDC. Thus, the total valve-overlap movement 34xe2x80x2 is about 20 degrees.
FIG. 1 depicts the valve overlap period 34xe2x80x2 creating an exhaust gas back-flow, which is drawn into the intake port 14xe2x80x2 via cylinder combustion chamber 26xe2x80x2. In prior art FIG. 2, the direction arrow 28xe2x80x2 indicates movement of the piston 24xe2x80x2 to a downward location from TDC, wherein the exhaust valve has closed and a low velocity air/fuel charge in-flows into the cylinder via the intake valve 20xe2x80x2. The low velocity intake charge results in incomplete mixing of the air and fuel, causing poor combustion stability.
The effects of poor in-cylinder mixing may be reduced by injecting fuel into the intake port during a period when the intake valve is closed. Such direct injection, however, results in extensive port wall-wetting, causing difficulty in controlling the cylinder air/fuel ratio during throttle transients, resulting in poor catalytic converter efficiency. While it is difficult to calibrate fuel compensation during cold-starting transients because of port wall-wetting, it will be noted that such wetting is an order of magnitude higher during cold-starts than under fully warmed-up conditions.
A feature of the invention is to provide an automotive engine variable valve control cold-start system wherein the intake valve opening is delayed a predetermined period after the exhaust valve has closed with both valves within the piston intake stroke region. Thus, both the intake and exhaust valves are closed during a predetermined crank-angle movement in the piston stroke region. This period, which is defined by the inventors as the xe2x80x9cnegativexe2x80x9d valve-overlap (NVO) period, functions to improve atomization and vaporization of the air/fuel charge entering the cylinder in a manner to be explained below.
Another feature of the invention is that as the piston descends during a first portion of its intake stroke, the NVO period starts, wherein a high-vacuum is created in the cylinder. On completion of the NVO period, the intake valve starts its valve-lift phase by starting to open, creating a slight gap. The pressure difference across the low-lift of the intake valve achieves a high velocity in-flow through the valve gap into the cylinder. Such a high velocity flow increases turbulence causing increased mixing of the air/fuel charge in the chamber, thereby improving combustion stability, which allows a leaner mixture together with an increase in spark retardation during cold-starts. The spark retard operates to raise the exhaust gas temperature, achieving rapid catalyst warm-up for reducing tailpipe emissions.
In conventional engines, air/fuel ratio excursions during throttle transients result in poor catalyst efficiency. With NVO, however, the increased mixing induced by the air/fuel charge high turbulence allows open-valve port fuel injection timing to be used under cold-start conditions. Open-valve injection minimizes the amount of port wall-wetting, thereby reducing air/fuel ratio excursions caused by throttle transients, improving catalyst warm-up efficiency during cold-starts.